be honest, not all doctors have good knowledge about breast feeding, unless they have their own experience or read up a lot about this aspect. They have not taught a lot about that in medical school.
However, it is true that the wound heal slower if it is always soiled with stool. On the other hand, breast feeding should not be interuppted.
The only reason your baby can an abscess in his buttock is that it always soil with stool, then it gets inflammation, wound and subsequently infection. It totally not related to your diet.
If you think you can keep your buttock always dry , you can continue breast feeding. It is easy to change the dressing yourself, pack with more gauze so stool will not sock the wound, change napkin 15 mins after each meal, check baby buttock hourly. It will heal gradually.
Read more breast feeding website and those patient materials written by medical professional (US or UK one), you will learn more and even know better than your doctor.